1. Field of the Invention
The invention relates to an estimated torque calculation device of an internal combustion engine and a method thereof. More particularly, the invention relates to a device and a method of precisely calculating an estimated torque generated by an internal combustion engine mounted in a vehicle.
2. Description of the Related Art
In conjunction with a vehicle equipped with an engine whose output torque can be controlled independently of the accelerator pedal operation of a driver and with an automatic transmission, there is a concept of “driving power control” that the positive/negative target driving torque calculated on the basis of the accelerator pedal operation amount of a driver, the operating condition of the vehicle, etc., is realized by the engine torque and the speed change gear ratio of the automatic transmission. Control techniques called “driving power request type”, “driving power demand type”, “torque demand system”, etc. are also similar to this concept.
The aforementioned torque demand-system engine control device calculates a target torque of the engine on the basis of the accelerator operation amount, the engine rotation speed, and an external load, and controls the amount of fuel injection and the amount of supply air.
In this torque demand-system engine control device, in reality, a target generated torque is calculated by adding to the requested output torque a loss load torque, such as a friction torque, that forms the loss in the engine or the power train system, and the amount of fuel injection and the amount of supply air are controlled so as to realize the target generated torque.
According to this torque demand-system engine control device, improvements in the drivability, such as the ability to always maintain a constant operating feeling, etc., can be achieved by using the torque of the engine, which is a physical amount that directly affects the control of the vehicle, as a reference value of the control.
In the torque demand-system engine control device as described above, the torque generated from the engine is used as a target value to control the engine. Therefore, it is important how to estimate the torque generated from the engine.
Japanese Patent Application Publication No. JP-A-2005-120886, which relates to an air-fuel ratio control, discloses a control device of an internal combustion engine which highly accurately calculates an actual graphically-determined torque that is one of parameters that reflect the actual air-fuel ratio. This control device of an internal combustion engine includes an actual graphically-determined torque calculation portion that calculates an actual graphically-determined torque on the basis of information about the state of combustion during idle operation, a reference air-fuel ratio graphically-determined torque calculation portion that calculates a reference air-fuel ratio graphically-determined torque occurring at a reference air-fuel ratio on the basis of information about a physical amount of supply to the internal combustion engine during idle operation, and an estimated air-fuel ratio calculation portion that calculates an estimated value of the actual air-fuel ratio (hereinafter, referred to as “estimated air-fuel ratio”) on the basis of the actual graphically-determined torque and the reference air-fuel ratio graphically-determined torque.
According to this control device of an internal combustion engine, during idle operation, the actual graphically-determined torque can be calculated on the basis of the information about the state of combustion since the vehicle drive system is not driven. The thus-calculated actual graphically-determined torque serves as a parameter that accurately reflects the actual air-fuel ratio of the mixture actually burned in the cylinders. Therefore, the use of the actual graphically-determined torque and the reference air-fuel ratio graphically-determined torque makes it possible to find a relationship the actual air-fuel ratio and the reference air-fuel ratio, from which the actual air-fuel ratio can be estimated (i.e., an estimated air-fuel ratio can be calculated). In this case, as a concrete calculation method for the actual graphically-determined torque, an actual graphically-determined torque may be found by calculating a mechanical friction loss on the basis of information about the engine rotation speed and the engine temperature (e.g., the cooling water temperature), and calculating a pumping loss on the basis of the intake pipe pressure, and calculating an external load torque on the basis of the state of operation of accessories, and summing the mechanical friction loss, the pumping loss and the external load torque. During idle operation, the actual graphically-determined torque is a value obtained by summing the internal loss torque (the mechanical friction loss and the pumping loss) and the external load torque (the load torque of accessories such as the compressor of the air-conditioner and the like). Therefore, the mechanical friction loss, the pumping loss, the external load torque can be accurately calculated, and the actual graphically-determined torque obtained by summing these values can be accurately calculated.
However, in a relatively high rotation speed region, the intake system and the combustion system are relatively stable, and therefore the state of operation of the internal combustion engine stabilizes. On the other hand, in a low rotation speed region, the state of operation of the internal combustion engine does not stabilize, due to the tendency for the intake system and the combustion system to become unstable and furthermore, the intervention of the ISC (Idle Speed Control) and the increases in the load output of the internal combustion engine. Therefore, even if the torque of the internal combustion engine is estimated by factoring in the losses and the external load in the low rotation speed region, there is possibility of deterioration of the estimation accuracy in the low rotation speed region.
However, the aforementioned Japanese Patent Application Publication No. JP-A-2005-120886 does not mention the accuracy deterioration regarding the estimated torque of the internal combustion engine in the low rotation speed region.